This invention relates in general to an optical apparatus for generating a finger image and more particularly to such an apparatus which provides an image with improved ridge and valley definition.
A number of techniques are known for optically reading a finger surface to provide an image which is the equivalent of an inked fingerprint. These techniques include a direct finger imaging technique described in U.S. Pat. No. 4,787,742 issued Nov. 29, 1988 and entitled Direct Finger Reading and the use of a curved platen in order to take into account the effect of the maximum angle of refraction on a platen as shown in U.S. Pat. No. 4,783,167 issued Nov. 8, 1988 and entitled Finger Imaging Apparatus Using Curved Platen.
The desired result is a fingerprint image with the ridges and valleys clearly shown so that ridge and valley end points and bifurcations can be identified to provide unique identification points for each finger. These identification points are called minutia and they provide a unique identification of the individual involved.
A major problem is that the ridges contain cracks and holes which create a non-continuous, non-smooth ridge line that tends to make automatic identification of the line ending and line bifurcation minutia more difficult. Complex mathematical techniques can be employed to heal some of these ridge defects. However, both for the purpose of visual examination of the image generated as well as from the point of view of automatic identification point determination, it is desirable to provide an image in which these ridge defects are minimized.
Another problem is that certain valleys close down under pressure of a finger on a platen thereby losing some definition and some minutia.
Accordingly, it is a major purpose of this invention to provide a technique in which the image of the ridges will bridge over the cracks, holes and discontinuities in the ridges as much as possible so as to provide a simplified fingerprint image for viewing and processing and in which the valleys will be held open so that a distinctive ridge/valley image can be formed.